Nowadays, certain automotive vehicles are provided with a device allowing the recharging by induction of a battery of a user apparatus such as, for example, a mobile phone. The term “battery” is understood to mean in the present document an electrical energy storage unit for powering the user apparatus.
Such a recharging device comprises a management module, an emission module and a reception surface for the user apparatus. The emission module comprises at least one antenna of induction emitting coil type which is coupled to the reception surface and which generates an electromagnetic field around the reception surface in a so-called “emission” zone, by operating for example in a frequency band lying between 100 and 200 kHz. The management module of the device is configured to control the emission module, in particular the power level of the emission by the coil, so as to detect a variation of voltage across the terminals of the emitting coil and to exchange messages with the user apparatus.
The user apparatus comprises, in a complementary manner, a control module and a receiver module. The receiver module comprises at least one antenna of induction receiving coil type which detects an electromagnetic field generated by the recharging device when it is placed in the emission zone of said device. The control module of the user apparatus is configured to control the reception module and exchange messages with the recharging device. An exemplary magnetic-induction-based power transfer system is known from the specification defined by the WPC consortium (the acronym standing for “Wireless Power Consortium”), known by the name “Qi low power specification” (version 1.1 of March 2012), incorporated by reference, which defines in particular the exchanges of messages between the recharging device and the user apparatus.
In a known manner, the electromagnetic field generated by the emitting coil induces an AC current in the receiving coil which makes it possible to recharge the battery of the user apparatus.
Thus, when the user wishes to electrically recharge the battery of his apparatus, he places it on the device's reception surface, thereby varying the voltage across the terminals of the emitting coil of the device.
The management module of the device detects this voltage variation and deduces therefrom that an at least partly metallic object has been placed on the reception surface.
The management module of the device then dispatches a recognition message to ascertain whether the object placed on the surface is a user apparatus compatible with the recharging device, that is to say which can be recharged by the device, or else another object such as an incompatible user apparatus or any other at least partly metallic object.
Hence, when the user apparatus is compatible with the device, its control module responds to the device by dispatching a compatibility message and the recharging by induction of the user apparatus starts until it is finished or until the user apparatus exits the emission zone.
When the user apparatus is incompatible with the device, its control module responds to the device by dispatching an incompatibility message and no electromagnetic field emission is carried out by the emission module of the device.
When the object is not an electrically rechargeable user apparatus or is any other at least partly metallic object such as a coin for example, no response message is received by the management module of the device subsequent to its emission of the recognition message and no electromagnetic field emission is initiated by the emission module of the device.
Such a message exchange thus makes it possible to limit access to recharging solely to compatible user apparatuses. Indeed, the generation of an electromagnetic field on a metallic object, for example of coin type, can lead to a significant rise in its temperature, for example greater than 80° C., and this may present risks of burns to the user.
Hereinafter, “parasitic object” will refer to an at least partly metallic object liable to heat up sufficiently to present a danger to the user when subjected to an electromagnetic field generated by the emitting coil of the device.
Such a danger can thus arise when a user apparatus and a parasitic object are present simultaneously in the emission zone during recharging of the user apparatus, thus presenting a significant drawback.
In order to ensure that there is no parasitic object present in the emission zone, the WPC protocol proposes to calculate the difference between the power emitted by the emitting coil and the power received by the receiving coil and to compare this difference with a predetermined threshold, for example 1 W.
This calculation is carried out at the level of the management module of the device which already knows the emission power of the emitting coil. It is therefore necessary for the device to receive the value of power received by the user apparatus. This information can be dispatched in a known manner by the user apparatus, as is for example described in the WPC protocol.
When the difference between the power emitted and the power received is smaller than the predetermined threshold, the management module of the device deduces therefrom that the object placed on the reception support is a user apparatus whilst if the difference between the power emitted and the power received is greater than the predetermined threshold, the management module of the device deduces therefrom that a parasitic object, which absorbs a great deal of energy, is present in the emission zone of the device.
This solution presents drawbacks however. Indeed, to limit the power emitted by the emitting coil, it is necessary for the emitting coil and the receiving coil to be perfectly aligned, i.e. superposed. By way of example, an emission power of 7 W may suffice to obtain a reception power of 5 W when the emitting coil and the receiving coil are perfectly aligned. On the other hand, when the emitting coil and the receiving coil are not aligned, this being frequent with a recharging device for automotive vehicle for which the user is content to place his user apparatus on a reception surface devoid of positioning means, the power received may be appreciably reduced, for example by 80% of the value of the emission power.
In order to solve this problem, the management module of the device uses in a known manner the information in respect of power received dispatched by the user apparatus and controls the emission module so that it increases the emission power of the emitting coil. Thus, still by way of example, it may be necessary to increase the emission power to 15 W to obtain a reception power of 5 W when the emitting coil and the receiving coil are not aligned.
However, in the case where the emitting coil and the receiving coil are not aligned, the difference between the power emitted and the power received is greater than the predetermined threshold mentioned previously so that the management module of the device may incorrectly deduce therefrom that a parasitic object is present in the emission zone of the device, thus presenting a significant drawback.
It is also known to detect a metallic parasitic object, by measuring the variation of the quality factor of the emitting coil and the variation of the voltage across the terminals of said coil. By comparing, the measured values, with predetermined thresholds, it is possible to detect the presence of a metallic parasitic object.
However, the variation of the quality factor of the emitting coil, and the variation of the voltage across the terminals of said coil in the simultaneous presence of a metallic parasitic object and of said apparatus (for example a coin placed under the apparatus) are dependent on the type of user apparatus, on its dimensions, on its composition (metallic, plastic), and on the characteristics of the receiving coil. This detection method is therefore not robust, and does not allow reliable detection since it is not possible to fix a unique predetermined detection threshold in order to detect a metallic parasitic object for all types of user apparatus inclusive.